Yan Xiang , Kärnenergiteknik, nuclear power safety
Wednesday 01 November
09:00 - 12:00
To assess corium coolability during severe accidents of LWR, significant number of studies have been conducted previously to investigate the characteristics of debris beds formed from FCI due to fragmentation of molten corium and settlement of the fragments in a water pool. However, their focus has been placed on the oxidic component (UO2/ZrO2) of corium, while little attention has been paid to the debris bed formation due to FCI of the metallic component (Zr/Fe) of corium. The present thesis work is motivated to fill the knowledge gap in debris bed formation, since metallic melt-coolant interactions may be encountered at the initial corium discharge following melt-through of RPV, especially in Nordic BWRs. The goal is to characterize the debris beds in a water pool through melt-coolant interaction experiments using various simulants of corium metallic component. A numerical simulation of melt jet breakup in a water pool is also performed. The thesis work consists of five parts.
The first part is the methodology about the experimental approach on metallic melt water interactions. A simplified scaling analysis is achieved based on the similarity of dimensionless numbers: Stephan number (St) and Froude number (Fr). As a result, simulant materials were selected, and DEFOR-M and DEFOR-O facility designs for metallic and oxidic melt respectively were determined. The facilities are equipped with high-speed cameras for visualization, weight sensors for melt flow rate and debris bed mass determination, three-dimensional laser scanner for debris bed profile measurement, as well as measurements of bed porosity and particle size distribution. The double-crucible design is used in DEFOR-O facility to avoid melt contamination by crucible which was found in the previous DEFOR tests.
The second part is an experimental investigation on debris bed formation in the case of a metallic melt jet falling into a water pool. DEFOR-M facility is employed in the experiment, with a focus on the influential parameters such as melt superheat, melt jet free fall height, coolant subcooling, water pool depth and material. Sn and Zn are used as simulants of metallic corium. The experimental results reveal mainly debris bed characteristics including configuration and porosity of a debris bed as well as morphology and size distribution of debris particles under various combinations of the influential parameters.
The third part is an extension of the second part by using the binary (Sn-Bi) alloy, to simulate the possible binary Zr-Fe mixture in corium. The test facility and procedure are the same as in the second part. It is found that a substantial fraction of Sn-Bi melt forms nonparticulate debris chunks in a highly subcooled water pool. The melt superheat has mild influences on the metallic melt coolant interactions under high water subcooling conditions, while the water subcooling has strong effects on solidification of melt droplets. The composition of the binary alloy also shows an influence on the melt water interaction process.
The fourth part is a comparison of debris bed characteristics obtained from different melt materials such as Sn, Sn-Bi, Zn and Bi2O3-WO3. The DEFOR-O facility with double crucibles is employed to perform debris bed formation experiment of Bi2O3-WO3 melt. The experimental results show that the melt materials have strong effects on debris bed characteristics under a comparable test condition. Metallic melts are sensitive to oxidation, especially in the Zn case where significant oxidic layers are found on the particles. Typical metallic particles are flake-like, with smooth surface and large aspect ratio. In contrast, particles in oxidic melt case have smooth surfaces with quasi-spherical shape and small aspect ratio. Coolant subcooling plays an important role in both oxidic and metallic melt-water interactions. Overall, bed porosity and debris particles in the DEFOR-M tests with metallic melt are much larger than those in the DEFOR-O tests with the oxidic melt and in the FARO tests with prototypical materials.
The fifth part is a numerical simulation of melt jet breakup in a water pool using a MCFD approach where a coupled CLSVOF method is used to capture melt-coolant interfaces. The focus is placed on the prediction of interface instabilities and jet breakup length, and their influential factors such as melt materials, jet diameter, free fall height, in-pool structures, multiple jets and pitch/diameter ratio. DEFOR-M tests are simulated by the numerical approach, and the comparative results show a good agreement between simulation and experiment, in terms of instability pattern and jet breakup length. It is also found that the jet breakup length in DEFOR-M tests cannot be predicted by the existing correlations (e.g., Taylor’s, Epstein & Fauske’s and Matsuo’s). Based on present experimental and numerical data, a new correlation for jet breakup length is proposed with a similar form of Satio’s correlation.